The invention relates generally to continuous casting.
More particularly, the invention relates to the continuous casting of metals, especially steel.
In the continuous casting of steel, molten steel is teemed into one end of a mold having opposite open ends, and a strand consisting of a solidified shell and a molten core is withdrawn from the other end of the mold. Upon exiting from the mold, the strand is sprayed with water, or a mixture of water and air, in order to solidify the molten core.
The great majority of continuous casting machines currently in use have vertical molds, and the molten steel is teemed into the upper end of the mold while the continuously cast strand is withdrawn from the lower end of the mold. The strand follows a downward path which, depending upon the type of machine, may be straight or curve towards the horizontal. In either event, significant pressure develops inside the strand when the cross section of the latter is large. This is due to the fact that the rate at which heat can be removed from a strand is limited so that, for a large strand, the core remains molten for a substantial distance from the mold. The pressure generated by the molten core, which may be referred to as the metallostatic pressure, is a function of the specific gravity of the steel being cast and the head of the molten steel, that is, the vertical distance from the mold.
In order to counteract the metallostatic pressure, continuous casting machines for large strands have roller aprons which receive the strand as it issues from the mold. The roller aprons, which consist of two series of rollers located on opposite sides of the strand, generally extend from the mold to at least that point along the path of the strand where the molten core has solidified completely. The cooling medium is sprayed onto the strand through the spaces between neighboring rollers. The roller aprons also perform a guiding function.
One of the drawbacks of conventional continuous casting machines for large strands resides in that the roller apron adds significantly to the cost and complexity of the machine. An important factor in the cost and complexity of the roller apron is the requirement that it be able to withstand the substantial metallostatic pressure of the molten core. Furthermore, although the roller apron counteracts the metallostatic pressure at the points of contact between the rollers and the strand, the metallostatic pressure causes bulging of the strand in the spaces between neighboring rollers. Accordingly, as the strand travels along the roller apron, the shell of the strand bulges outwards in the spaces between neighboring rollers and is subsequently pushed inwards by the rollers. The alternating stresses generated in this manner may lead to defects in the strand.
Conventional continuous casting machines, including machines for small strands, also have a host of drawbacks related to the spray cooling of the strand. One such drawback is that homogeneous cooling is very difficult to achieve. Non-homogeneous cooling is another source of defects in continuously cast strands. Furthermore, non-homogeneous cooling results in distortion of the geometric shape of the strand.
Another drawback related to spray cooling is that the spray nozzles tend to become clogged. This not only enhances non-homogeneous cooling of the strand but adds to maintenance costs.
Moreover, the surface of the stand oxidizes during spray cooling. The scaling which accompanies oxidation results in loss of material. Oxidation also adversely affects surface quality so that a surface layer of the strand must be removed after casting when good surface quality is required, e.g. when casting certain steel grades. This not only entails a further loss of material but increases labor costs as well. In addition, the scale generated during spray cooling must be periodically removed from the machine which again adds to maintenance costs.
Since the water used for spray cooling becomes contaminated, a water treatment system is necessary. Such a system not only constitutes a substantial capital expenditure but also represents a significant maintenance item.